Title: Spin excitations used to probe the nature of exchange coupling in the magnetically ordered ground state of Pr 0.5 Ca 0.5 MnO 3

We have used time-of-flight inelastic neutron scattering to measure the spin wave spectrum of the canonical half-doped manganite Pr 0.5Ca 0.5MnO 3 in its magnetic and orbitally ordered phase. Comparison of the data, which cover multiple Brillouin zones and the entire energy range of the excitations, with several different models shows that only the CE-type ordered state provides an adequate description of the magnetic ground state, provided interactions beyond nearest neighbor are included. We are able to rule out a ground state in which there exist pairs of dimerized spins which interact only with their nearest neighbors. The Zener polaron ground state, which comprises strongly bound magnetic dimers, can be ruled out on the basis of gross features of the observed spin wave spectrum. A model with weaker dimerization reproduces the observed dispersion but can be ruled out on the basis of subtle discrepancies between the calculated and observed structure factors at certain positions in reciprocal space. Adding further neighbor interactions results in almost no dimerization, i.e. interpolating back to the CE model. These results are consistent with theoretical analysis of the degenerate double exchange model for half-doping.

@article{osti_1295094,
title = {Spin excitations used to probe the nature of exchange coupling in the magnetically ordered ground state of Pr0.5Ca0.5MnO3},
author = {Ewings, R. A. and Perring, T. G. and Sikora, O. and Abernathy, D. L. and Tomioka, Y. and Tokura, Y.},
abstractNote = {We have used time-of-flight inelastic neutron scattering to measure the spin wave spectrum of the canonical half-doped manganite Pr0.5Ca0.5MnO3 in its magnetic and orbitally ordered phase. Comparison of the data, which cover multiple Brillouin zones and the entire energy range of the excitations, with several different models shows that only the CE-type ordered state provides an adequate description of the magnetic ground state, provided interactions beyond nearest neighbor are included. We are able to rule out a ground state in which there exist pairs of dimerized spins which interact only with their nearest neighbors. The Zener polaron ground state, which comprises strongly bound magnetic dimers, can be ruled out on the basis of gross features of the observed spin wave spectrum. A model with weaker dimerization reproduces the observed dispersion but can be ruled out on the basis of subtle discrepancies between the calculated and observed structure factors at certain positions in reciprocal space. Adding further neighbor interactions results in almost no dimerization, i.e. interpolating back to the CE model. These results are consistent with theoretical analysis of the degenerate double exchange model for half-doping.},
doi = {10.1103/PhysRevB.94.014405},
journal = {Physical Review B},
number = 1,
volume = 94,
place = {United States},
year = {2016},
month = {7}
}

While the spin dynamics of La 0.7Ca 0.3MnO 3 in the ferromagnetic phase are known to be unconventional, previous measurements have yielded contradictory results regarding the damping of spin wave excitations. Neutron spectroscopy measurements on a sample with a transition temperature of T c = 257 K, higher than most single crystals, unambiguously reveal an anomalous increase in spin wave damping for excitations approaching the Brillouin zone boundary along the [100] direction that cannot be explained as an artifact due to a noninteracting phonon branch. Spin waves throughout the ( HK0) plane display a common trend where the spin wavemore » damping is dependent upon the excitation energy, increasing for energies above roughly 15 meV and reaching a full width at half maximum of at least 20 meV. In conclusion, the results are consistent with a model of intrinsic spatial inhomogeneity with phase separated regions approximately 18 Å in size persisting over a large range of temperatures below T c.« less

The perovskite Co oxidesmore » $${\mathrm{Pr}}_{1{-}x}{\mathrm{Ca}}_{x}{\mathrm{CoO}}_{3}$$ exhibit at around $$x{\sim}0.5$$ an unusual transition at temperature $${T}_{\mathrm{S}}$$ [S. Tsubouchi et al. Phys. Rev. B 66, 052418 (2002)] with no space group change and no long-range magnetic order. We measured inelastic neutron scattering intensities of magnetic excitations $$I({Q},{\omega}$$) for two single crystals of ($${\mathrm{Pr}}_{1{-}y}{\mathrm{Y}}_{y}{)}_{1{-}x}{\mathrm{Ca}}_{x}{\mathrm{CoO}}_{3}$$ in which the $x$ region of the transition is widened by the Pr-site doping to study whether the recent proposal of the excitonic condensation model is relevant to this "hidden order transition" of the system. While $${{\chi}}^{{'}{'}}({Q},{\omega})$$ seems to have characteristics of strongly correlated Co $3d$ electrons above $${T}_{\mathrm{S}}$$, it abruptly exhibits a weak feature reminiscent of a pseudogaplike structure at $${T}_{\mathrm{S}}$$ with decreasing $T$. The first peak of the Pr crystal-field excitations has also been observed at low temperatures. Using a model of the coexistence of the exciton condensed phase (EC phase) with the nearly or weakly ferromagnetic one, we show that the EC phase appears as the collective transition in $${\mathrm{Pr}}_{1{-}x}{\mathrm{Ca}}_{x}{\mathrm{CoO}}_{3}$$.« less

Evolution of the average and local crystal structure of Ca-doped LaMnO 3 has been studied across the metal to insulator (MI) and the orthorhombic to rhombohedral (OR) structural phase transitions over a broad temperature range for two Ca concentrations (x=0.18,0.22). Combined Rietveld and high real space resolution atomic pair distribution function (PDF) analysis of neutron total scattering data was carried out with aims of exploring the possibility of nanoscale phase separation (PS) in relation to MI transition, and charting the evolution of local Jahn-Teller (JT) distortion of MnO 6 octahedra across the OR transition at T S~720 K. The studymore » utilized explicit two-phase PDF structural modeling, revealing that away from TMI there is no evidence for nanoscale phase coexistence. The local JT distortions disappear abruptly upon crossing into the metallic regime both with doping and temperature, with only a small temperature-independent signature of quenched disorder being observable at low temperature as compared to CaMnO 3. The results hence do not support the percolative scenario for the MI transition in La 1₋xCa xMnO 3 based on PS, and question its ubiquity in the manganites. In contrast to LaMnO 3 that exhibits long-range orbital correlations and sizable octahedral distortions at low temperature, the doped samples with compositions straddling the MI boundary exhibit correlations (in the insulating regime) limited to only ~1 nm with observably smaller distortions. In the x=0.22 sample local JT distortions are found to persist across the OR transition and deep into the R phase (up to ~1050 K), where they are crystallographically prohibited. Lastly, their magnitude and subnanometer spatial extent remain unchanged.« less

In this paper, we present inelastic neutron-scattering experiments on the S=1/2 frustrated gapped quantum magnet piperazinium hexachlorodicuprate (PHCC) under applied hydrostatic pressure. These results show that at 9 kbar the magnetic triplet excitations in the system are gapless, contrary to what was previously reported. Our results are in agreement with recent muon-spin relaxation experiments which found magnetic order above a quantum-critical point at 4.3 kbar. Finally, we show that the changes in the excitation spectrum can be primarily attributed to the change in a single exchange pathway.